1. Field of the Invention
The present invention relates to a CDMA/TDD-based transmission/reception apparatus carrying out open-loop transmit power control and its transmit power control method.
2. Description of the Related Art
A CDMA (Code Division Multiple Access) system is one of multiple access systems by which a plurality of stations in a radio transmission system such as car telephones and cellular telephones carry out communications on a same frequency band simultaneously and transmits information signals with their spectrum spread over a band wide enough compared to the original bandwidth, having the features of achieving highly efficient use of frequencies and accommodating many users.
However, it has a near-far problem when each mobile station transmits a signal by the same power on the CDMA system and a desired transmission station is far and a non-desired transmission station (interference station) is near. The near-far problem is the reception power of a signal transmitted from the interference station becomes greater than the reception power of a signal transmitted from the desired transmission station, failing to suppress mutual correlation between spreading codes with a processing gain alone, which leads to make communications impossible.
Therefore, a cellular system using the CDMA system requires transmit power control according to the conditions of each channel on the uplink. It also requires transmit power control for compensating against instantaneous fluctuations of received signal power as fading which is a cause of deterioration of the line quality in terrestrial mobile communication.
Here, there are cases where a TDD (Time Division Duplex) is used as a duplex system for this multiple access communication system. The TDD system is a system carrying out communications by time-sharing a same radio frequency for transmission and reception, and since it uses a same frequency band for transmission and reception its frequency correlation of fading fluctuations between a transmission signal and reception signal is 1.
Furthermore, if the period of switching between transmission and reception is short enough, time correlation in the condition of the channel of mutual fading fluctuations, etc. is high, and therefore mobile stations can perform open-loop transmit power control that controls transmission power based on received signal power effectively.
If the base station has a plurality of antennas, there are cases where transmission diversity is used which selects the most suitable transmission antenna based on received signal power of those antennas. Using transmission diversity eliminates the necessity of space diversity at mobile stations making it possible to reduce the size of those mobile stations.
The following is an explanation of a base station and mobile station of a CDMA/TDD-based radio transmission system which carries out conventional open-loop transmit power control and uses transmission diversity, using the attached drawings.
FIG. 1 is a block diagram showing a configuration of a conventional base station. The base station apparatus shown in FIG. 1 comprises modulation section 11 that modulates transmission data, spreading section 12 that multiplies the modulated signal by spreading code A and spreads it, antenna control section 13 that switches transmission antennas, antennas 14 and 15 that transmit/receive signals, despreading section 16 that multiplies the received signal by spreading code B and despreads it, demodulation section 17 that demodulates the despread signal and antenna selection section 18 that measures the level of the received signal from the demodulation result and selects a transmission antenna.
The transmission data are modulated by modulation section 11 and spread with spreading code A by spreading section 12. Then, the spread signal is transmitted from either antenna 14 or antenna 15 controlled by transmission antenna control section 13.
The signal received by antenna 14 and antenna 15 is despread by despreading section 16 with spreading code B. The despread signal is demodulated by demodulation section 17 with received data extracted and the demodulation result input to transmission antenna selection section 18 as the antenna selection information. Then, transmission antenna selection section 18 compares the levels of the received signals at the two antennas based on the demodulation result and the antenna that received the larger level is selected as the antenna for transmitting data in the next slot and the signal showing the selection result is output to transmission antenna control section 13.
FIG. 2 is a block diagram showing a configuration of a conventional mobile station. The mobile station shown in FIG. 2 comprises antenna 21 that transmits/receives signals, despreading section 22 that multiplies the received signal by spreading code A and despreads it, demodulation section 23 that demodulates the despread signal, received signal power measuring section 24 that measures the level of the received signal from the demodulation result, modulation section 25 that modulates transmission data, spreading section 26 that multiplies the modulated signal by spreading code B and spreads it and transmit power control section 27 that performs transmit power control based on received signal power.
The signal received by antenna 21 is despread by despreading section 22 with spreading code A, demodulated by demodulation section 23 with the received data extracted and the demodulation result input to received signal power measuring section 24. Received signal power measuring section 24 measures received signal power from the demodulation result and the measurement result is input to transmit power control section 27. Transmit power control section 27 calculates a transmission power value from the transmission power value of the base station, target received signal power value at the base station and measurement result.
The transmission data are modulated by modulation section 25, spread by spreading section 26 with spreading code B, and with power amplified by transmit power control section 27 based on the calculated transmission power value, transmitted from antenna 21.
Thus, in the conventional radio transmission system the base station transmits signals by selecting one antenna from a plurality of antennas and the mobile station carries out open-loop transmit power control based on received signal power.
However, in the conventional radio transmission system above, the mobile station carries out transmit power control only targeted at the antenna through which the base station transmitted signals and if signals are received by a plurality of antennas at the base station, transmission power is not controlled for the antennas that did not transmit signals and transmission power is not enough controlled for all of reception antennas, causing a problem of causing transmit power control errors.
It is an objective of the present invention to reduce transmit power control errors to a small level when the base station performs reception through a plurality of antennas.
The present invention achieves the objective above by the base station spreading the transmission data distributed by the number of transmission antennas with mutually different spreading codes, then transmitting them in parallel from a plurality of transmission antennas, and by the mobile station despreading the each of received signals transmitted the plurality of transmission antennas, measuring and combining received signal power and controlling transmission power based on the combined received signal power.